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  <div class="section" id="numpy-lexsort">
<h1>numpy.lexsort<a class="headerlink" href="#numpy-lexsort" title="Permalink to this headline">¶</a></h1>
<dl class="function">
<dt id="numpy.lexsort">
<code class="sig-prename descclassname">numpy.</code><code class="sig-name descname">lexsort</code><span class="sig-paren">(</span><em class="sig-param">keys</em>, <em class="sig-param">axis=-1</em><span class="sig-paren">)</span><a class="headerlink" href="#numpy.lexsort" title="Permalink to this definition">¶</a></dt>
<dd><p>Perform an indirect stable sort using a sequence of keys.</p>
<p>Given multiple sorting keys, which can be interpreted as columns in a
spreadsheet, lexsort returns an array of integer indices that describes
the sort order by multiple columns. The last key in the sequence is used
for the primary sort order, the second-to-last key for the secondary sort
order, and so on. The keys argument must be a sequence of objects that
can be converted to arrays of the same shape. If a 2D array is provided
for the keys argument, it’s rows are interpreted as the sorting keys and
sorting is according to the last row, second last row etc.</p>
<dl class="field-list simple">
<dt class="field-odd">Parameters</dt>
<dd class="field-odd"><dl class="simple">
<dt><strong>keys</strong><span class="classifier">(k, N) array or tuple containing k (N,)-shaped sequences</span></dt><dd><p>The <em class="xref py py-obj">k</em> different “columns” to be sorted.  The last column (or row if
<em class="xref py py-obj">keys</em> is a 2D array) is the primary sort key.</p>
</dd>
<dt><strong>axis</strong><span class="classifier">int, optional</span></dt><dd><p>Axis to be indirectly sorted.  By default, sort over the last axis.</p>
</dd>
</dl>
</dd>
<dt class="field-even">Returns</dt>
<dd class="field-even"><dl class="simple">
<dt><strong>indices</strong><span class="classifier">(N,) ndarray of ints</span></dt><dd><p>Array of indices that sort the keys along the specified axis.</p>
</dd>
</dl>
</dd>
</dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference internal" href="numpy.argsort.html#numpy.argsort" title="numpy.argsort"><code class="xref py py-obj docutils literal notranslate"><span class="pre">argsort</span></code></a></dt><dd><p>Indirect sort.</p>
</dd>
<dt><a class="reference internal" href="numpy.ndarray.sort.html#numpy.ndarray.sort" title="numpy.ndarray.sort"><code class="xref py py-obj docutils literal notranslate"><span class="pre">ndarray.sort</span></code></a></dt><dd><p>In-place sort.</p>
</dd>
<dt><a class="reference internal" href="numpy.sort.html#numpy.sort" title="numpy.sort"><code class="xref py py-obj docutils literal notranslate"><span class="pre">sort</span></code></a></dt><dd><p>Return a sorted copy of an array.</p>
</dd>
</dl>
</div>
<p class="rubric">Examples</p>
<p>Sort names: first by surname, then by name.</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">surnames</span> <span class="o">=</span>    <span class="p">(</span><span class="s1">&#39;Hertz&#39;</span><span class="p">,</span>    <span class="s1">&#39;Galilei&#39;</span><span class="p">,</span> <span class="s1">&#39;Hertz&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">first_names</span> <span class="o">=</span> <span class="p">(</span><span class="s1">&#39;Heinrich&#39;</span><span class="p">,</span> <span class="s1">&#39;Galileo&#39;</span><span class="p">,</span> <span class="s1">&#39;Gustav&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ind</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">lexsort</span><span class="p">((</span><span class="n">first_names</span><span class="p">,</span> <span class="n">surnames</span><span class="p">))</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ind</span>
<span class="go">array([1, 2, 0])</span>
</pre></div>
</div>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="p">[</span><span class="n">surnames</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">+</span> <span class="s2">&quot;, &quot;</span> <span class="o">+</span> <span class="n">first_names</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">ind</span><span class="p">]</span>
<span class="go">[&#39;Galilei, Galileo&#39;, &#39;Hertz, Gustav&#39;, &#39;Hertz, Heinrich&#39;]</span>
</pre></div>
</div>
<p>Sort two columns of numbers:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">4</span><span class="p">]</span> <span class="c1"># First column</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">b</span> <span class="o">=</span> <span class="p">[</span><span class="mi">9</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">1</span><span class="p">]</span> <span class="c1"># Second column</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ind</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">lexsort</span><span class="p">((</span><span class="n">b</span><span class="p">,</span><span class="n">a</span><span class="p">))</span> <span class="c1"># Sort by a, then by b</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ind</span>
<span class="go">array([2, 0, 4, 6, 5, 3, 1])</span>
</pre></div>
</div>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="p">[(</span><span class="n">a</span><span class="p">[</span><span class="n">i</span><span class="p">],</span><span class="n">b</span><span class="p">[</span><span class="n">i</span><span class="p">])</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">ind</span><span class="p">]</span>
<span class="go">[(1, 0), (1, 9), (3, 0), (4, 1), (4, 2), (4, 4), (5, 4)]</span>
</pre></div>
</div>
<p>Note that sorting is first according to the elements of <code class="docutils literal notranslate"><span class="pre">a</span></code>.
Secondary sorting is according to the elements of <code class="docutils literal notranslate"><span class="pre">b</span></code>.</p>
<p>A normal <code class="docutils literal notranslate"><span class="pre">argsort</span></code> would have yielded:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="p">[(</span><span class="n">a</span><span class="p">[</span><span class="n">i</span><span class="p">],</span><span class="n">b</span><span class="p">[</span><span class="n">i</span><span class="p">])</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">np</span><span class="o">.</span><span class="n">argsort</span><span class="p">(</span><span class="n">a</span><span class="p">)]</span>
<span class="go">[(1, 9), (1, 0), (3, 0), (4, 4), (4, 2), (4, 1), (5, 4)]</span>
</pre></div>
</div>
<p>Structured arrays are sorted lexically by <code class="docutils literal notranslate"><span class="pre">argsort</span></code>:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([(</span><span class="mi">1</span><span class="p">,</span><span class="mi">9</span><span class="p">),</span> <span class="p">(</span><span class="mi">5</span><span class="p">,</span><span class="mi">4</span><span class="p">),</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">0</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span><span class="mi">4</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="mi">0</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span><span class="mi">2</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span><span class="mi">1</span><span class="p">)],</span>
<span class="gp">... </span>             <span class="n">dtype</span><span class="o">=</span><span class="n">np</span><span class="o">.</span><span class="n">dtype</span><span class="p">([(</span><span class="s1">&#39;x&#39;</span><span class="p">,</span> <span class="nb">int</span><span class="p">),</span> <span class="p">(</span><span class="s1">&#39;y&#39;</span><span class="p">,</span> <span class="nb">int</span><span class="p">)]))</span>
</pre></div>
</div>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">np</span><span class="o">.</span><span class="n">argsort</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="c1"># or np.argsort(x, order=(&#39;x&#39;, &#39;y&#39;))</span>
<span class="go">array([2, 0, 4, 6, 5, 3, 1])</span>
</pre></div>
</div>
</dd></dl>

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